MoS2 and WS2 are quasi two dimensional (2D) compounds. Atoms within a layer are bound by strong covalent forces, while individual layers are held together by van der Waals (vdW) interactions. The stacking sequence of the layers can lead to the formation of a hexagonal polymorph with two layers in the unit cell (2H), rhombohedral with three layers (3R), or trigonal with one layer (1T). The weak interlayer vdW interactions offer the possibility of introducing foreign atoms or molecules between the layers via intercalation. Furthermore, MoS2, WS2 and a plethora of other 2D compounds are known to form closed cage structures which are referred to as inorganic fullerene-like (IF) and inorganic nanotubes (INT), analogous to structures formed from carbon [1]. One of the initial methods of synthesis of IF-MoS2 and IF-WS2 involved starting from the respective oxide nanoparticles [2, 3]. Subsequently synthesis of IF-NbS2 and IF-MoS2 using a gas-phase reaction starting from MoCl5 and NbCl5, respectively, and H2S has been demonstrated [4a, 5]. A similar strategy for the synthesis of IF-MoS2 nanoparticles using the gas phase reaction between Mo(CO)6 and sulfur, has been reported [4b]. The two kinds of reactions progress along very different paths, which have a large effect on the topology of the closed-cage nanoparticles. The conversion of the metal-oxide nanoparticles to sulfides starts on the surface of the nanoparticles progressing gradually inwards in a slow diffusion-controlled fashion. Contrarily, the gas-phase reaction proceeds by a nucleation and growth mode starting from, a small, e.g. MoS2, nuclei and progressing outwards rather rapidly.
Modification of the electronic properties of layered-type semiconductors can be accomplished by doping/alloying process of the semiconductor, in which metal atoms go into the semiconductor layer, substituting the host transition metal or the chalcogene atoms. If the substituting atom (e.g. Nb) has one less electron in its outer shell than the host metal atom (Mo), the lattice becomes p-doped. If the substituting metal atom has one extra electron (Re), the lattice becomes n-type. If the substituent atom is chlorine, replacing the sulfur atom, the nanoparticle becomes n-type. Doping is usually limited to below 1 at % substitution. In the case of alloying, the guest atoms are of significant concentrations (>1%), in which case if the percolation limit is surpassed (e.g. Mo0.75Nb0.25S2), the lattice becomes essentially metallic.
Alloying or doping of inorganic nanotubes has been reported for specific cases of Ti-doped MoS2 nanotubes, Nb-doped WS2 nanotubes [6(a),(b)]. In addition, W-alloyed MoS2 nanotubes have been synthesized by varying the W:Mo ratio [6(c)]. However, since there was not much control of the amount of foreign atoms in the lattice, control of the electronic properties of the nanoparticles could not be achieved in the previous works.
Control of the doping level in nanoparticles, and in particular in inorganic nanotubes and fullerene-like structures can lead to various unique phenomena. Addition of various nanoparticles to boost tribological performance (i.e., reduced friction and wear) of lubricating fluids has been under investigation for sometime [7]. Equally important is the need for replacing the current oil-additives, in the search for environmentally more benign counterparts. Using semiconducting nanoparticles, like MoS2 and WS2 or the respective selenides as additives to lubricating fluids provides a unique tool to understand the electronic component of friction and wear. Thus, there arises the need for a new synthetic strategy of using inorganic fullerene-like (IF) nanoparticles and inorganic nanotubes (INT) of semiconductors, doped with metal and non-metal atoms.